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Commit 848ac114 authored by Miklos Szeredi's avatar Miklos Szeredi Committed by Al Viro
Browse files

vfs: check submounts and drop atomically 



We check submounts before doing d_drop() on a non-empty directory dentry in
NFS (have_submounts()), but we do not exclude a racing mount.

 Process A: have_submounts() -> returns false
 Process B: mount() -> success
 Process A: d_drop()

This patch prepares the ground for the fix by doing the following
operations all under the same rename lock:

  have_submounts()
  shrink_dcache_parent()
  d_drop()

This is actually an optimization since have_submounts() and
shrink_dcache_parent() both traverse the same dentry tree separately.

Signed-off-by: default avatarMiklos Szeredi <mszeredi@suse.cz>
CC: David Howells <dhowells@redhat.com>
CC: Steven Whitehouse <swhiteho@redhat.com>
CC: Trond Myklebust <Trond.Myklebust@netapp.com>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: default avatarAl Viro <viro@zeniv.linux.org.uk>
parent db14fc3a

fs/dcache.c

+65 −0
Original line number Diff line number Diff line
@@ -1264,6 +1264,71 @@ void shrink_dcache_parent(struct dentry *parent) 
}

EXPORT_SYMBOL(shrink_dcache_parent);



static enum d_walk_ret check_and_collect(void *_data, struct dentry *dentry)

{

	struct select_data *data = _data;



	if (d_mountpoint(dentry)) {

		data->found = -EBUSY;

		return D_WALK_QUIT;

	}



	return select_collect(_data, dentry);

}



static void check_and_drop(void *_data)

{

	struct select_data *data = _data;



	if (d_mountpoint(data->start))

		data->found = -EBUSY;

	if (!data->found)

		__d_drop(data->start);

}



/**

 * check_submounts_and_drop - prune dcache, check for submounts and drop

 *

 * All done as a single atomic operation relative to has_unlinked_ancestor().

 * Returns 0 if successfully unhashed @parent.  If there were submounts then

 * return -EBUSY.

 *

 * @dentry: dentry to prune and drop

 */

int check_submounts_and_drop(struct dentry *dentry)

{

	int ret = 0;



	/* Negative dentries can be dropped without further checks */

	if (!dentry->d_inode) {

		d_drop(dentry);

		goto out;

	}



	for (;;) {

		struct select_data data;



		INIT_LIST_HEAD(&data.dispose);

		data.start = dentry;

		data.found = 0;



		d_walk(dentry, &data, check_and_collect, check_and_drop);

		ret = data.found;



		if (!list_empty(&data.dispose))

			shrink_dentry_list(&data.dispose);



		if (ret <= 0)

			break;



		cond_resched();

	}



out:

	return ret;

}

EXPORT_SYMBOL(check_submounts_and_drop);



/**

 * __d_alloc	-	allocate a dcache entry

 * @sb: filesystem it will belong to

include/linux/dcache.h

+1 −0
Original line number Diff line number Diff line
@@ -253,6 +253,7 @@ extern void d_prune_aliases(struct inode *); 


/* test whether we have any submounts in a subdir tree */

extern int have_submounts(struct dentry *);

extern int check_submounts_and_drop(struct dentry *);



/*

 * This adds the entry to the hash queues.